This invention relates generally to high speed impact printers and, more particularly, to a circuit for varying the hammer firing times to compensate for variations in velocity as a character band moves along its path.
Good print registration is highly desirable in achieving quality printing. One of the more common difficulties in maintaining registration is the slight variation in velocity of the type carrying member in high speed printers. This variation, due to poor drive motor regulation or constantly changing impact loading by varying numbers of print hammers, alters the point at which a released hammer impacts its selected character on the type carrier. The flight time of a hammer or impact member after release is relatively constant, but a selected type element can be misplaced to a readily noticeable degree by velocity changes during the hammer flight time when band speeds are nominally hundreds of inches per second. A variation in the nominal band velocity of even one to three percent of nominal velocity results in easily descernible misregistration.
The usual solutions have been to measure the velocity of the type carrying element to determine the velocity error, converting this to a time period, then delaying the hammer firing time by that amount. These measurements required the movement of many type characters past an optical or magnetic sensing transducer so that corrections were determined infrequently and were incapable of immediate effectiveness.
One example of this type of approach is described in the U.S. Pat. No. 3,974,765 in which the elapsed time for the passage of a plurality of type character synchronizing marks is measured and summed with accumulated clock pulses. The accumulated pulses are then compared with the value that should be accumulated if the carrier were going at its nominal velocity. Any difference in count between the actual and nominal totals is then decremented by other clock pulses to provide a delay that is used for the next plurality of characters while a new compensation delay is calculated.
An article from the IBM Technical Disclosure Bulletin entitled "Digital Correction of Hammer Firing Times" by J. H. Meier and J. W. Raider, Volume 14, No. 12, May 1972, pages 3565-6, teaches a similar approach by again using a series of character synchronizing marks to increment clock pulses into an up-down counter. This establishes a value indicative of the belt velocity which switches on a synchronizing signal to cause the counter to decrement downward to zero from the same clock and produce a hammer fire signal. Changes in the maximum count reached, of course, vary accordingly the time to count down to zero.
These techniques also require the provision of an adjustable delay circuit for each hammer. In line printers, this adds significantly to the cost and complexity of the circuits because of the large number of hammers that need to be controlled. If the velocity of the type character band is to be changed radically to provide multiple print speeds, these circuits are not readily adapted to accommodate the change because of the limited ranges of the hammer delay times of which they are capable.